Once Abu Bakrah (RAA) came late for Salah and found the Prophet (SAAW) and Sahabah already in Ruku’ position. Not wanting to miss a raka’t of Salah he started his Salah before he joined the line still walking (while in Ruku) and then joined the line. The Prophet (PBUH) heard his breathing, and afterwards when he knew what had happened he said: “May Allah increase your eagerness (caring to join Salah) but do not come back (doing it again.)” Hadith narrated by Bukhari.

The mistakes Abu Bakrah made were three:
1. Rushing to Salah
2. Performing Ruku’ outside the line of Salah
3. Walking while in Ruku’ to Salah

Walking a step or two while in prayer is permissible for a legitimate reason. For example, Sahabah walked and turned around when Qiblah was changed during the prayer. The prophet (SAAW) allowed Muslims to kill harmful insects during prayer and that would require sometimes walking a step or so. Abu Bakr walked backwards while leading Salah to allow the prophet (PBUH) to step up and lead. Scholars allowed Muslims to walk forward to fill in the gaps in lines for prayers, etc.

Concerning the issue of starting Salah alone behind an established line of Salah (while space is available), Imam Ahmad and Ishaq said it is prohibited and the prayer must be redone. The majority of scholars including Abu Hanifa, Malek and Shafei said it is (Makrooh) not preferred, but not necessary for the Salah to be redone. They supported their point of view by the above mentioned Hadith that the prophet (PBUH) did not tell him to redo Salah. However, if there is no space left to join in the line, it is permissible to start a new line alone.