Fiqh-us-Sunnah Volume 5, Hajj and ‘Umrah, Fiqh 5.105B.

Delay in throwing 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. So she and Safiyah (her mother) were left behind, and they arrived in Mina after sunset on the 10th of Zhul-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 aforementioned hadith of Ibn ‘Abbas. 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.