It is narrated from Sayyiduna Zayd bin Arqam (may Allah be pleased with him); he said: The Messenger of Allah (peace and blessings be upon him) passed by the Quba' mosque while they were praying two rak'ahs, and he (peace and blessings be upon him) asked: "What prayer is this?" The Companions said: "Tasbih (that is, a voluntary prayer)." He said: "The prayer of the oft-repentant (al-awwabin) is when the feet of the young camels begin to burn."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5090