The following are four examples of such use of the word rasul ('messenger') in Sahih Bukhari:
1. In a long narration in Bukhari, a Companion of the Holy Prophet, Ka‘b ibn Mālik, relates:
“When forty out of the fifty nights elapsed, then the rasul of the rasul of Allah came to me…”
— Book: Expeditions (Maghāzī), hadith number 4418.
Here a messenger sent by the Holy Prophet is described as his rasul.
2. During his last illness the Holy Prophet sent a message to Hazrat Abu Bakr to lead the prayer, as he was too weak to come out. It is related:
“The messenger (rasul) went to Abu Bakr and said: The Messenger of Allah (rasul-ullah) orders you to lead the people in the prayer.”
— Book: Call to Prayer (Adhān), hadith number 687.
Again a messenger sent by the Holy Prophet is called a rasul.
3. A man describes his meeting with Hazrat Uthman when the latter was Khalifa. He says:
“Then the messenger (rasul) of Uthman came and I went to him (i.e., to Uthman)…”
— Book: Excellences of the Companions (Faḍā’il Aṣḥāb), hadith number 3696.
Here a messenger sent by a Muslim ruler is called a rasul.
4. There is a famous incident that the Holy Prophet, shortly after taking up his mission, climbed the Safa mountain and called upon all the clans of the Quraish to assemble around him. He asked them if they would believe him if he were to tell them that he could see an enemy on its way to attack them, and they replied that they would believe him as they knew him to be truthful. It is stated in hadith relating to this event:
“Any clan which could not come sent its messenger (rasul) to see what it was.”
— Book: Commentary on the Quran, in explanation of 26:214, hadith number 4770.
Evidently, these observers sent by the clans, each observer being called a rasul, were unbelievers.
The following is an example from the Hadith collection Abu Dawud. The event mentioned in this hadith is very well known.
“When the Messenger of Allah decided to send Mu‘adh [ibn Jabal] to Yaman [as Governor], he asked him how he would decide cases. Mu‘adh replied: ‘By the Book of Allah’. He asked: ‘But if you do not find [any direction] in it’. He replied: ‘Then by the practice (Sunnah) of the Messenger of Allah’. ‘But if you do not find [any direction] in the Sunnah’, he asked. ‘Then I will exercise my judgment and spare no effort’, Mu‘adh replied.
The Messenger of Allah said: ‘Praise be to Allah Who has granted the messenger (rasul) of His Messenger (rasul) what pleases the Messenger of Allah (rasul-ullah) ’.”
— Abu Dawud, book: Office of the Judge (al-aqḍiyah), ch. 11, hadith number 3592.
Here the Holy Prophet himself has described a Companion, whom he is sending, as rasul of the rasul of Allah.
— Zahid Aziz