The Messenger of Allah (sal Allahu alaihi wa sallam) said: “Sleeping in the morning hinders sustenance.” [Majma uz Zawa’id]
The practice of the Messenger of Allah (sal Allahu alaihi wa sallam) and his Companions, after praying Fajr, was to remain sitting in the place where they had prayed, engaged in the remembrance of Allah (subhana wa ta’ala), until the sun rose. When the sun rose they would get up, talk about things, and laugh and smile. [Sahih Muslim]
Allah (subhana wa ta’ala) commands us in the Quran to engage in His remembrance especially before sunrise and sunset.
For this reason, many of the early Muslims disliked the idea of sleeping after Fajr. Urwah ibn al-Zubayr reported that his father Al-Zubayr bin al-Awwam (radi Allahu anhu) used to forbid his children from sleeping in the morning. Urwah said: “I do not hear of any man who sleeps in the morning but I lose interest in that person.” [Ibn Abi Shaybah in Musannaf]
It is not Haram to sleep after Fajr or Asr but we miss out on Allah’s treasure if we do so. May Allah (subhana wa ta’ala) grant us the ability to maximize on the blessings of this time.