[Q]: O eminent shaykh, a man praying Ṣalāt al-Ẓuhr drops his handkerchief while he is standing so he bends down and grabs the handkerchief, is his prayer invalidated by this movement?

[A]: Yes, his prayer is invalidated by this movement; because when he makes rukūʿ he bends down until he reaches the point of the rukūʿ, but he has (now) increased the rukūʿ (i.e. gone past the rukūʿ point to grab his handkerchief or added another rukūʿ). However, if he has done so out of ignorance then there is nothing upon him due to the generality of His, Exalted be He, statement:

رَبَّنَا لَا تُؤَاخِذۡنَآ إِن نَّسِينَآ أَوۡ أَخۡطَأۡنَاۚ

“Our Lord, punish us not if we forget or fall into error.”
[Sūrah al-Baqarah 2: 286]

Due to this, if you drop a handkerchief or key while you are standing in prayer then leave it until you reach it in sujūd or pick it up with your foot. If you are able to stand on one foot, then get it with your foot then grab it (from your foot) with your hand. As for the person bending down to grab it from the ground wherein the rukūʿ would be closer to it than the qiyām, then this is not permissible.

Source: sahab.net
Translated by: Raha ʿAzīzuddīn Batts