If any small portion (or) hair is left dry unknowingly and is noticed after performing Wudu or Ghusl, then is it enough to wet that part only or should I repeat them?

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

If the water does not reach any part of the skin during obligatory Ghusl, then it is mandatory to wash that part only as washing in sequence is not a condition in Ghusl. But in ablution, one should wash that part and all the organs that usually follow it in sequence. If, for instance, a part of the arm was left dry, it becomes obligatory to wash that part and all the remaining organs; i.e., one should wipe on his hair and wash his feet. If a person prays with an uncomplete ablution or Ghusl, then his prayer is not valid and he has to repeat it.

For more benefit on the continuation when performing ablution, please refer to Fatwa 139853.

For more benefit on obligatory acts that must be washed in ablution, and which are detailed in sequence order, please refer to Fatwa 326854.

For more benefit on the nullifiers of ablution, please refer to Fatwa 84093.

For more benefit on the detailed description of ablution, please refer to Fataawa 82845 and 83164.

For more benefit on obligatory and Sunnah acts of ablution and prayer, please refer to Fatwa 84841.

Allah knows best.