The prayer of the oft-returning to Allah is when weaned camels are bitten by excessive heat
Authentic hadithعن زَيد بن أرْقَم رضي الله عنه : أنه رأى قوما يصلُّون من الضُّحى، فقال: أمَا لقد عَلِموا أن الصلاة في غير هذه السَّاعة أفضل، إن رسول الله صلى الله عليه وسلم ، قال: «صلاة الأَوَّابِين حين تَرْمَضُ الفِصَال».
Zayd ibn Arqam (may Allah be pleased with him) reported that he saw some people pray in the forenoon, so he said: "They know that praying at an hour other than this one is better, for the Messenger of Allah (may Allah's peace and blessings be upon him) said: 'The prayer of the oft-returning to Allah is when weaned camels are bitten by excessive heat.'"
Explanation
Zayd ibn Arqam (may Allah be pleased with him) saw some people pray in the forenoon, so he said that he heard the Prophet (may Allah's peace and blessings be upon him) say that the prayer of the oft-returning to Allah is when the heat is so scorching that the hooves of little camels burn on the ground. That is the best time to pray the Duha prayer for those who obey Allah and often return to Him.
Source · Narrated by Muslim
From the Hadith Encyclopedia (hadeethenc.com).