1a] How did Allaah open the chest of His noble Messenger Muhammad (peace and blessings

1a]
because the narrators are trustworthy and well known. End quote.
The incident of his chest being opened is also mentioned in some
reports at other times, at the age of ten, and when his mission began,
but these are weak reports. See: al-Seerah al-Nabawiyyah al-Saheehah
(1/103).
Ibn Katheer said in Tafseeral-Qur'aan il-'Azeem (4/677):
Allaah says "Have We not opened your breast for you (O Muhammad)?"
meaning, did We not open your heart for you, i.e., We illuminated it
etc. This is like the verse in which Allaah says (interpretation of
the meaning): "And whomsoever Allaah wills to guide, He opens his
heart to Islam." [al-An'aam 6:125].
And it was said that what is meant by the verse "Have We not opened
your breast for you (O Muhammad)?" is the opening of his chest on the
night of the Isra', but there is no contradiction because the opening
of his chest includes what was done on the night of the Isra' as well
as the metaphorical meaning of opening his heart. And Allaah knows
best. End quote.
3 – It says in Rooh al-Ma'aani (30/166):
It was said that what is meant is: Did We not relieve your distress
and worry by showing you the reality of things and the insignificance
of this world, so that enabled you to bear hardship by mans of turning
to Allaah with du'aa'.
It was narrated from the majority that what is meant is: Did We not
bestow upon him wisdomand enable him to receivethat which was revealed
to him after it was difficult for him?
4 – Ibn 'Ashoor said in al-Tahreer wa'l-Tanweer (1/4850):
Opening his breast is a metaphor for the blessings that were bestowed
upon him and enabling him to attain all levels of perfection, and
informing him that Allaah is pleased with him, and glad tidings of the
victory that the religion would attain. End quote.
See: Subul al-Huda wa'l-Rashaad (2/59).
5 – Shaykh Ibn 'Uthaymeen (may Allaah have mercy on him) said in
Tafseer Soorat al-Sharh (p.1):
This opening is metaphorical, not physical. The opening of the heart
may be done so that it is able to receive the decree of Allaah of both
types, the shar'i decrees of Allaah, which means religion, and the
decree of Allaah which has to do with the calamities that may befall
man. End quote.
And Allaah knows best.