During Salaatu at-Tasbeeh, I happen to say only ten Subhanallah Walhamdulillah Wala Ilaha Illallah Wallahu Akbar instead of the fifteen after reciting Fatihah and a Surah. How do I make for the rest of the five?

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.

Tasbeeh (saying Subhana Allah) in the prayer of Tasbeeh is not obligatory. So whoever leaves it or leaves a number of Tasbeeh, does not have to do anything to compensate for that and his prayer is valid, and he does not have to perform prostrations of forgetfulness. He does not also have to pray this prayer again.

Ibn Hajar said in his Fataawa: “Tasbeeh in this prayer takes the ruling of Takbeer in the ‘Eed prayers. So the praying person does not have to prostrate for forgetting any of them (Tasbeeh or Takbeer) even if he initially intended a given number and did not fulfill it. His prayer is valid.”

For more benefit, please refer to Fatwa 189987, 83877, and 89834.

Allah knows best.