Al-Islam, as a complete way of life, gives us directions with regards to our day-to-day activities and dealings. Sitting is part of our lives and as mundane as it is, Islam does not fail to guide us unto doing it correctly. These guides are essential to us, as they assist us in.
1. On the Grave
Abu Hurairah (may Allah be pleased with him) narrates that the Prophet (ﷺ) said: "If one of you were to sit on a hot coal that burns him, that would be better for him than if he were to sit on a grave."
[Sunan Ibn Majah 6/1633]
2. Inbetween two people except by their permission
This is courtesy. You should try to avoid discomforting people. Abdullah ibn Amr ibn al-'As (may Allah be pleased with him) narrated that RasuluLlah (ﷺ) said: "One should not sit between two men except with their permission.
3. In the Masjid, until you offer two units of prayer (Tahiyyatul Masjid)
This is loved (i.e. mustahabb). Abu Qatadah (may Allah be pleased with him) narrated that RasuluLlah (ﷺ) said: "When one of you comes to the Masjid, then let him not sit until he has performed two.
4. Under the Sun or Inbetween Sun & Shade
Ibn Abbas (may Allah be pleased with them) narrated that the Prophet (ﷺ) was preaching & a man stood in the sun, so he asked him to enter the shade [Abu Dawud 3300]. Ibn Buraydah (may Allah be pleased with him) narrated that the Prophet.
5. By the Roadside
The Prophet (ﷺ) forbade sitting by the Roadside except for those who can fulfill the rights of the road, which are to:
• Lower one's gaze
• Refrain from harming others
• Return greetings
• Enjoin good
• Forbid evil
[Sahih al-Bukhari 6229]
6. While a Funeral Procession is Passing by
Abu Sa'eed Al-Khudri (may Allah be pleased with him) narrated that the Prophet (ﷺ) said: "Whenever you see a funeral (procession), stand up (for it) & whoever follows it, then do not sit down until it is put down."
7. With a Person that has communicable diseases
Abu Huraira (may Allah be pleased with him) narrated that the Prophet (ﷺ) said: "Do not put a patient with a healthy person" (as a precaution). Therefore excessive closeness to someone who has a communicable disease is…
8. With a Manifest Transgressor (fāsiq)
This is a person who makes his wrobe of pride the rejection of Islamic injunctions. Sitting with him has to be for reminder puroses, or else stay away. Allah says "if you see those who oppose our verses, turn away from them" [Q6:68].
9. On a Table Where Alcohol is Served
Sayyiduna Umar (may Allah be pleased with him) said that the Prophet (ﷺ) said: "whoever believes in Allah and the last day should not sit on a table where alcohol is served".
[Musnad Ahmad 125]
10. Sitting with a Fool
Well, Obviously because foolishness is communicable. You either get foolish too, or at least get rated like a fool. Sayyiduna Ali (may Allah be pleased with him) said:
فلا تصحب أخا الجهـ … ـل وإياك وإياه.
فكم من جاهل أردى … حكيما حين آخاه.
يقاس المرء بالمرء … إذا ما المرء ماشاه.