Meanwhile, an envoy came hurriedly from Shaam, and asked to meet Qutuz, the leader of the Islamic forces, and the other emirs, saying that he was a messenger sent by Saarim Ad-Deen Aybak, one of the Muslim emirs captured by Hulagu when he conquered Shaam, and accepted to serve in the Tatar army, and took part with them in different battles, and came with them to ‘Ayn Jaaloot. However, we do not know whether he had accepted to cooperate with the Tatars willingly, or unwillingly, awaiting a good opportunity to benefit the Muslims; that is something between him and Allaah the Almighty.
However, what we know is that before the ‘Ayn Jaaloot Battle, he decided to serve the Muslim army as much as he could. Exalted be Allaah who says (what means): {And none knows the Soldiers of your Lord except Him.} [Quran 74:31]

Although this man was known neither to Qutuz nor to the emirs of the army, he was put by Allaah the Almighty in this place to serve the Muslims.
Let us observe the Arrangement of the Lord of the Worlds; when the Muslim forces arrived in Palestine, and Kitbuqa moved towards ‘Ayn Jaaloot, Saarim Ad-Deen Aybak sent that messenger to Qutuz, to tell him very important information about the army of the Tatars.

This messenger transmitted to Qutuz, May Allaah Have mercy upon him, the following information:
First, the army of the Tatars is not as strong as it used to be, since Hulagu took many leaders and soldiers with him to Tabriz in Persia. Thus, the army no longer had the same status as when it entered Shaam. So, do not feel afraid of it.
This information was indeed of great importance. It raised the spirits of those who feared the Tatars. Of course, you could find in this army some who might talk to themselves or to each other about the previous wars of the Tatars in which they had destroyed the countries and exterminated their people. For this reason, this information was a very important impetus that provided the Muslims with further power.

Second, the right wing of the Tatar army is stronger than its left wing, which requires the Muslim army to strengthen its left wing more which would face the right wing of the Tatars.

Third, is a very important piece of information: Al-Ashraf Al-Ayyoobi, of Homs, along with his brigade, as well as Saarim Ad-Deen Aybak, would join the Tatar army, but they would seem to be defeated in front of the Muslims. In other words, Al-Ashraf Al-Ayyoobi reconsidered his situation and preferred to side with the army of Qutuz, and he set out with the Tatarian army just to deceive the Tatars and break their unity.

Such information was very important, and came just in time. However, it was out of wisdom that the Muslims did not depend upon this news, since it might have been a trick made by the Tatars, Al-Ashraf Al-Ayyoobi or Saarim Ad-Deen Aybak.

For this reason, the emirs said to each other, with great wisdom and outstanding military experience: "In no way should this be something wrought (by the Tatars) to deceive the Muslims."
Nevertheless, the Muslims were careful, and benefitted from that news with no indulgence in the preparation, nor negligence in precaution and wariness.
That was the end of Ramadhaan 24th.
The Muslims spent that night standing (in prayer), imploring, supplicating and hoping.
It was one of the greatest nights of the year, since it was one of the last ten nights of Ramadhaan.
It was, moreover, an odd night, and might probably have been the Night of Al-Qadr.

It was, also, one of the greatest nights of this World, since it preceded the day of Jihaad, and on its morning, the Muslims would take retaliation for the blood of millions of Muslims which was shed at the hands of the barbarous Tatars.
That is, my brothers and sisters, really an eternal night.

https://www.islamstory.com/en/artical/20741/muslim-leader-join-tatars