With recourse to history, we come to know that Imam al-Husayn (as) gave two types of permission to his companions.
A. General permission
Imam al-Husayn (as) addressed his companions in a general way,
«أمّا بعد؛ فانّي لا اعلم اصحاباً اوفی ولا خيراً من اصحابي، ولا اهل بيت ابرّ واوصل من اهل بيتي، فجزاکم الله عنّي خيراً. ألا وانّي لأظنّ يومنا من هولاء غداً. ألا وانّي قد اذنت لکم، فانطلقوا جميعاً في حلّ، ليس عليکم حرج منّي ولا ذمام، هذا الليل قد غشيکم فاتخذوه جملاً.»
"And after this; verily, I have never known companions more loyal and better than my companions. I have never found household members more excellent in observation of bonds of relationship than my Household (the Ahl al-Bayt). Allah will reward you graciously on my behalf. Beware! I strongly predict that the day of our confrontation with them will be tomorrow. Beware! I have given you permission to leave. You are all free to go. I do not and will not reproach anyone of you for doing so. This is a night whose darkness has covered you like a mounted camel. Therefore, take advantage of it and make your escapes."1
B. Special or personal permission
History narrates that Imam al-Husayn (as) did not content himself with the general permission he had given to his companions. He went further and gave personal and special permission to some people for certain reasons.
Imam al-Husayn (as) addressed Muhammad ibn Bushr Hadrami on the night of ‘Ashura in this way, "Your child is being held captive in Marzari." Muhammad ibn Bushr replied, "I entrust his captivity and mine to Allah and His account. I would rather die than live to see my child in captivity. I would not desire to remain alive after him."
When Imam al-Husayn (as) heard this, he said, "May Allah have mercy on you. You are free from any obligation to me. Go and strive hard to free your child. You are not obliged to me at all." He answered,
"May the wild beasts tear me to pieces alive if I should leave you." Imam al-Husayn (as) gave him some clothes and said, "Give these clothes to your son so that he may exert himself to free his brother." The value of those clothes was one thousand dinars.2
Imam al-Husayn (as) came out of the tents in the heart of the night to examine the pits and holes which were in the desert. Nafi‘ ibn Bilal followed the Holy Imam (as). Imam al-Husayn (as) asked him, "Where are you going?" Nafi‘ ibn Bilal answered, "O son of Allah's Prophet! Your movement towards this rebellious army has aroused my fears."
The Holy Imam (as) said, "I came out of my tent in order to examine the highs and lows of this desert so as to be able to distinguish the enemy's military strategic position." Imam al-Husayn (as) took Nafi‘ ibn Bilal's hand into his hands and said, "Verily, it is exactly as I have already predicted. I swear upon Allah!
The promise cannot be abrogated." After this he said, "Are you not going to take advantage of the darkness and pass through these two hills in the heart of the night in order to save yourself?"
Nafi‘ ibn Bilal fell at Imam al-Husayn's (as) feet and started kissing them. He said, "May my mother mourn for me! My sword is worth a thousand dinars and my horse is worth a hundred dinars. I swear upon Allah who holds me indebted for the favor of your presence! I will never leave you."3
1. Muqarram, Maqtal al-Husayn (as), p. 212.
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