Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear questioner, we would like to thank you for the great confidence you place in us, and we implore Allah Almighty to help us serve His cause and render our work for His Sake.
Answering your question, The Permanent Committee for Scholarly Research and Ifta’, issued the following fatwa:
One of the conditions of wudu’ (ablution) is washing all the parts in close sequence. Thus, if a person forgets to wash a limb required in wudu’, or even a small part of it, and remembers it while performing wudu’ or immediately after when the washed parts are still wet, they may wash the part they forgot and the subsequent limbs only.
However, if they remember having forgotten to wash a limb that is required to be washed during wudu’, or even a small part of it, after it is dry, or while or after performing Prayer, they should repeat wudu’ as prescribed by Allah and perform the whole Prayer again.
This is because the condition of washing the required body parts in sequence was not met. Moreover, Allah made it obligatory that all the required body parts should be washed during wudu’.
Accordingly, whoever omits even a very small area of a limb, it is as if it were wholly omitted. This is supported by the Hadith narrated on the authority of `Umar ibn Al-Khattab (may Allah be pleased with him) who said: “The Prophet (peace be upon him) saw a person performing wudu’ (ablution), but he left a nail-size space on his feet (unwashed). The Messenger of Allah (peace be upon him) commanded him to repeat the wudu’ and Prayer. The man went back and repeated the prayer.” (Ibn Majah)
Allah Almighty knows best.