It is very important to note that by throwing pebbles, one emulates Prophet Ibrahim (peace and blessings be upon him) when Satan blocked his way at that place in order to cast doubts in his heart or tempt him and divert him away from his Lord, so Allah commanded Ibrahim to drive the Satan away by throwing pebbles at him.
Coming to this issue, it can be stated that delaying throwing the pebbles until after dark is permissible if there is a genuine excuse for doing so.
In this regard, we will cite what Sheikh Sayyed Sabiq states in his well-known book Fiqh As-Sunnah:
“Delaying throwing the pebbles until after dark is permissible if there is a genuine excuse for doing so as related by Malik from Nafi` that a daughter of Safiyah, wife of Ibn `Umar, gave birth to a baby in Muzdalifah. For this reason, she and Safiyah (her mother) were left behind, and they arrived in Mina after sunset on the 10th of Dhul-Hijjah. Ibn `Umar asked them to throw their pebbles when they arrived and he found no harm in their doing so.
It is disliked, however, to delay throwing the pebbles without a valid excuse until after dark. According to the Hanafi and Shafi`i schools, and in the light of a narration from Malik, there is no penalty for delaying the throwing of pebbles, considering the hadith of Ibn `Abbas that on the Day of Sacrifice the Prophet (peace and blessings be upon him) was told by a man at Mina that he had thrown his pebbles after sunset, whereupon he said to the man, “There is no harm.” (Al-Bukhari) Ahmad is of the opinion that if a pilgrim delays the throwing of pebbles till the end of the Day of Sacrifice, then he should not throw pebbles during the night, but should throw them the next afternoon.”