It is narrated from Abdullah bin Mas'ud (may Allah be pleased with him) that he was asked about the explanation of the verse «ولا تحسبن الذين قتلوا في سبيل الله أمواتا بل أحياء عند ربهم يرزقون» , so he said: O people! Listen, we asked (the Messenger of Allah, peace and blessings be upon him) about its explanation, and we were informed that the souls of the martyrs are in the form of green birds, roaming freely in Paradise wherever they wish, and in the evening they take shelter in lanterns hanging from the Throne. Once your Lord looked upon them and asked: "Is there anything you desire so that I may grant it to you?" They said: O Lord! We desire nothing more. We roam wherever we wish in Paradise. Then (one day) Allah looked upon them again and said: "Is there anything you desire so that I may grant it to you?" When they saw that (Allah was determined to give, and that) there was no escape without asking and receiving, they said: Return our souls to our bodies so that we may go back to the world and be killed again in Your path. Imam Tirmidhi says:
This hadith is Hasan Sahih.

With this chain, Ibn Abi Umar narrated from Ibn Mas'ud in the same manner, but with the addition: "Convey our greetings to our Prophet and also inform him that we are pleased and content with our Lord, and He is pleased and content with us." Imam Tirmidhi says:
This hadith is Hasan. (That is: according to the above chain, because there is a break in the other chain.)