Respected ‘Ulama is the following incident authentic and can it be used to prove tashkil:
In the battle of Uhud, Rasulullah (sallallahu’alayhi wasallam) made necessary arrangements for the watch and ward of the camp during the night and then enquired: “Now, who is going to guard my tent during the night?”
A person (standing at his place): “I, O Nabi of Allah!”
Rasulullah (sallallahu’alayhi wasallam): “What is your name?”
The person: “Zakwan.”
Rasulullah (sallallahu’alayhi wasallam): “All right. You take your seat.”
He again inquired: “Who else is volunteering to guard my tent for tonight?”
A voice: “I, O Nabi of Allah!”
Rasulullah (sallallahu’alayhi wasallam): “Who are you?”
A voice: “Abu Saba’ (father of Saba’).”
Rasulullah (sallallahu’alayhi wasallam): “All right. Sit down.”
He enquired for the third time: “Who will be the third man to guard my tent tonight?”
Again came a voice from the crowd: “I, O Nabi of Allah!”
Rasulullah (sallallahu’alayhi wasallam): “Your name’?”
The voice: “Ibn Abdul Qais (son of Abdul Qais).”
Rasulullah (sallallahu’alayhi wasallam): “All right. You also sit down.”
Then Rasulullah (sallallahu’alayhi wasallam) bade all the three volunteers to come to him. Only one person came forward. Rasulullah (sallallahu’alayhi wasallam): “Where are your other two comrades’?”
The Person: “O Nabi of Allah! It was I who stood up all the three times.
Rasulullah (sallallahu’alayhi wasallam) blessed him with his prayers and allowed him to guard his tent. He kept watching the tent all night long.
Answer
This incident is recorded by Imam ‘Abdullah ibnul Mubarak (rahimahullah) in his Kitabul Jihad, Hadith: 151 with a weak chain.
The support you are seeking is indeed found in this incident.
A more authentic Hadith however, will be the one in Sahih Bukhari, Hadith: 2510, wherein Rasulullah (sallallahu’alayhi wasallam) asked the Sahabah (radiyallahu’anhum):
‘Who is ready to go to ‘Ka’b ibn Ashraf…’
And Allah Ta’ala Knows best,
Answered by: Moulana Muhammad Abasoomar
Checked by: Moulana Haroon Abasoomar