Mukhtasar Muslim (Abridged) 368 — Salat al-Awwabeen

It is narrated from Qasim Shaybani that Sayyiduna Zayd bin Arqam (may Allah be pleased with him) saw some people performing prayer at the time of Duha, so he said: These people know well that prayer is more virtuous at a time other than this. The Noble Prophet (peace and blessings be upon him) said that "the prayer of the oft-returning (awabeen)" is at the time when the feet of the young camels begin to burn (i.e., when the sun has risen high and the heat has intensified).