Salat al-Awwabin (the prayer of the penitent) is when the young camels feel the heat of the hot sand Sahih Muslim 748 b
Translation
Zaid b. Arqam reported that the Messenger of Allah (ﷺ) went out to the people of Quba' and saw them observing prayer; upon this he said
The prayer of the penitent should be observed when the young weaned camels feel heat of the sun.