Prayer is a devotional act performed in various forms, each with its own rules and conditions. None can take the place of another: each has its own particular status and must be performed in accordance with its prescribed order and requirements. The five daily prayers, for example, are offered at specific times and with a set number of units and particular conditions; recommended prayers, supererogatory prayers, the funeral prayer, the prayer of circumambulation, and others each have their own requirements. Performing any one of them does not suffice in place of the others. Sending blessings upon Muhammad and his family is likewise, in a sense, a distinct prayer whose performance is obligatory, for through it the Exalted Lord is remembered in the finest manner.
Like the funeral prayer, sending blessings upon Muhammad and his family (peace be upon them) does not require ablution.
قَالَ فَأَخْبِرْنِي عَنْ صَلَاةٍ فَرِيضَةٍ تُصَلَّى بِغَيْرِ وُضُوءٍ وَ عَنْ صَوْمٍ لَا يَحْجُزُ عَنْ أَكْلٍ وَ لَا شُرْبٍ قَالَ أَمَّا الصَّلَاةُ بِغَيْرِ وُضُوءٍ فَالصَّلَاةُ عَلَى النَّبِيِّ وَ آلِهِ ص وَ أَمَّا الصَّوْمُ فَقَوْلُ اللَّهِ عَزَّ وَ جَلَّ- إِنِّي نَذَرْتُ لِلرَّحْمنِ صَوْماً فَلَنْ أُكَلِّمَ الْيَوْمَ إِنْسِيًّا
*Al-Iḥtijāj ʿalā ahl al-lijāj* (by al-Ṭabarsī), vol. 2, p. 329
Abū Baṣīr reports that Ṭāwūs al-Yamānī asked Imam al-Bāqir, peace be upon him: “Tell me of the obligatory prayer that is performed without ablution, and of the fast that does not keep a person from eating and drinking.” He replied: “As for the prayer without ablution, it is a prayer [of blessings] sent upon Muhammad and his family. As for that kind of fast, this verse speaks of it: إِنِّی نَذَرْتُ لِلرَّحْمَنِ صَوْمًا فَلَنْ أُکَلِّمَ الْیَوْمَ إِنسِیًّا”