What is it like to think of the Prophet in prayer?

It is not bad to think of the Holy Prophet (peace and blessings of Allah be upon him) in prayer, rather, when the worshiper recites "Assalamu Alaika Ayyuhan Nabiyyu" while reciting the Tashahhud and the meaning remains in his heart, then it is necessary to think of the Holy Prophet (peace and blessings of Allah be upon him) at that time; However, the "Sarf-e-Himmat" experiment prevalent among the Sufis is forbidden in prayer. 

Sarf-e-Himmat is an independent topic in Sufism,
In Sufi books, the mention of "Sarf-e-Himmat" (or Tasarruf-e-Himmat) comes in a special sense, and its meaning is somewhat broader than the general literal meaning.

What is "Himmat" to the Sufis?
What is meant by Himmat according to the people of Sufism?
That inner strength of the heart that is directed towards a goal with concentration and attention.
It is not just intention but a name for heartfelt attention, concentration and spiritual power.

Meaning of "Sarf-e-Himmat" or "Tasarruf-e-Himmat"
In the terminology of Sufism, two famous uses of it are found:
 To direct all one's inner attention towards Allah
That is, the seeker should turn his heart away from the world and focus completely on the pleasure and nearness of Allah Almighty.
This meaning is more common in educational and reformatory books.
For example:
The Sheikh advises his disciple to "unite your Himmat and dedicate it only to Allah."

 Spiritual attention of the Sheikh (Tasarruf-e-Himmat)
According to some Sufis, "Tasarruf-e-Himmat" means that Allah Almighty gives such acceptance to a saint that his prayer or attention causes reform or change in someone's condition.

But the important thing according to Ahl-e-Sunnat
The real effect is only from Allah Almighty.
The Himmat of the saint or Sheikh is not the real influencer itself, but becomes a reason by the permission of Allah.
If the belief arises at any point that the Sheikh has a permanent influence, then this is not correct.

In any case, the reason why thinking about the Holy Prophet (peace and blessings of Allah be upon him) has been called corrupting the prayer is that attention in prayer should only be towards Allah. If someone gets so lost in thinking about the Prophet that the essence of God is removed from his mind, then this is wrong, because it is against the purposes of prayer.

And Allah knows best.