Is saying "Allahumma Subhanaka (O Allah, glory be to You)" during prostration, before supplicating for the good of this world and the hereafter, legislated, or should one be limited to what is sunnah? And is it permissible to praise Allah, send blessings upon the Prophet, and make tawakkul (rely entirely on Allah) at this point after saying "Subhana Rabbiyal-A'la (Glory be to my Lord, the Most High)"?

The glorification (tasbih) in bowing (ruku') and prostration (sujud) is not obligatory according to the majority of scholars. It is prescribed for the worshipper to say in his prostration: "Subhanaka Allahumma Rabbana wa bihamdika Allahumma ighfir li" (Glory be to You, O Allah, our Lord, and with Your praise, O Allah, forgive me), as was reported from 'Aisha. It is also prescribed to praise Allah with "La uhsi thana'an 'alaika Anta kama athnaita 'ala Nafsika" (I cannot enumerate Your praise, You are as You have praised Yourself), and to say "Subbuhun Quddusun Rabbul Mala'ikati war-Ruh" (Glorified, Most Holy, Lord of the angels and the Spirit). Furthermore, sending blessings upon the Prophet, peace and blessings be upon him, is prescribed in prostration because it is a form of supplication. It is also prescribed to entrust matters to Allah through supplication, for prostration is a place where supplications are answered.