Is there any Saheeh hadith which says that angels will keep on writing good deeds for a person who had recited 'Bismillah wal Hamdulillah' b4 making ablution.. and angels will keep writing till ablution is nullified?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

After research, we have not come across a Hadeeth with this meaning. However, we know a Hadeeth narrated by Abu Hurayrah that the Prophet said: "The angels supplicate in favor of one of you so long as he remains in the place where he has performed the prayer and does not invalidate his ablution. They (the angels) say: 'O Allaah! Forgive him, O Allaah! Have mercy upon him. And a person continues to be in the prayer as long as there is nothing preventing him from going back to his family except the prayer." [Al-Bukhari]

So, this Hadeeth is about the angels supplicating for a praying person as long as he is sitting in his praying place and his ablution is not invalidated.

Allaah Knows best.