Sheikh
Saalih al Fawzaan حفظه الله mentions:
These
are the instances where extending the Salaam is disliked:
The First: It is disliked to
extend the Salaam to someone who is speaking, because he is occupied with
speaking to someone else.
The Second: “One who is busy
with Dhikr (verbal remembrance of Allaah)” such as Tasbeeh (SubhanAllaah),
Tahleel (La Ilaha IllAllaah) and the recital of the Qur'aan.
The Third: One who is busy with the
speech (Hadeeth) of Muhammad صلّى اللَّهُ عليه وسلّم.
The Fourth: Extending the
Salaam to one who is giving a Khutbah (a public speech), with the
exception of the Jum’ah (Friday) Khutbah, for it is prohibited to
speak during it; it is prohibited to extend the Salaam to the Khateeb or those
attending the Jum’ah prayer, this is not permissible.”
The Fifth: “and a person
giving a lesson” one who is giving a lesson in Qur'aan, Hadeeth, Fiqh or Arabic
grammar, do not extend the Salaam to the teacher. Some people come to a circle
of knowledge while the class is being taught and extend the Salaam with a
raised voice. This is disliked, because it distracts the teacher and those
sitting and listening, as they would now give their attention to the new comer.
The Sixth: “Those who are
engaged in knowledge based research.” One who is researching a specific
knowledge based issue, do not extend the Salaam to him and interrupt his
research.