The source of the excerpt is the Полное собрание русских летописей which is a multi-volume publication of Russian medieval chronicles, documents and stories that were collected and printed in the nineteenth century. More specifically, the excerpt comes from volume 10 (1885) which has the title летописный сборник именуемый патриарсшею или никоновскою летописю. This is the so-called Nikon or Patriarch's Chronicle. (See, in Russian, Никоновская летопись.) The chronicle, complied by monks, is a year by year listing of events that took place in Russia.
If you wish, you can have a look at the original Russian version (1885) that I've translated. Let me note that the version published in 1885 is a nineteenth-century transcription of the original chronicle, which was written in medieval church Slavonic, and believe me, there are a lot of differences between medieval Church Slavonic and nineteenth-century Russian. Furthermore, contemporary Russian is a bit different from nineteenth-century Russian--the alphabet was simplified after the 1917 Russian Revolution. Thus, translation of this excerpt is not easy:).
This excerpt comes from page 105 of the 1885 edition.
[Brackets] indicate insertions within quotations to clarify the meaning of the phrase.
(Parenthesis indicate notes about the passage).
Lastly, dates in the chronicle are given according to the Byzantine calendar, which was the calendar used in Russia until 1700 when Peter the Great ordered the calendar changed. So, the year in the chronicle is indicated as 6745, which was 1237.
Here are some other Russian medieval texts, that I have not yet had an opportunity to translate, dealing with the Mongols.
Batu Khan’s Army.
That very year, came from the eastern lands to Riazan’s lands and forests, the godless Tatars, with their tsar Batu, and the invaders made their first camp near Onoz (We have not been able to determine exactly where that was; see враг на рязанских границах.),
and captured it and burned it. And from there he sent an emissary to Grand Prince Iurii Riazanskii Ingvorovich (See Юрий Игоревич.) and to his brother Oleg Ingvorovich (See Олег Ингваревич Красный, who was not actually his brother.) and other Riazan princes, demanding tribute from each of them: princes, people, horses, armor. The Grand Prince Iurii Ingvorovich, and his brother prince Oleg Ingvorovich, and the Murom and Pronsk princes answered the emissary of Batu, saying,
“коли нас не будет, то все ваше будет”
(This idiomatic phrase presents a lot of difficulty in trying to translate it. Here are some options that I, and others, have come up with:
If we do not, then you will take all that is ours.
Since we will not be there, everything there will be yours.
If we do not [accede to your demands], then you will take all that is ours.
If we aren't there, (then) everything will be yours.
When we are gone, everything will be yours.
If you kill us, then all this will be yours.)
And thus the war began, and [the princes] went against [the Tatars] in Voronezh, wanting the battle to happen there, and to Grand Prince Iurii Vsevolodich (the prince of Vladimir; See Юрий Всеволодович.) a messenger was sent asking him to come and help against the godless tsar Batu, but Grand Prince Iurii Vsevolodich Volodimirskii did not come, and he did not send his forces, nor did he listen to the pleas of the Riazan princes as he wanted to go alone into battle.
But already the wrath of God was aroused, bewilderment, terror, fear, and trembling for our sins;wisdom, quick and powerful military deeds were called for, but into strong hearts crept feminine weakness, and among them not one of the Russian princes would help one another.
The Riazan, Murom and Pronsk princes went against the godless and engaged them in battle, but the battles went bad, and the godless Ishmaelites [Muslims] conquered the princes who fled to their own cities. The Tatars, completely angered, began to plunder Riazan territory with great savagery, ransacked their towns and cut down and enslaved the people.