After two days the people of Kufa sent about 150 letters, each of which was from one, two or four men. Most of these letters were brought from Kufa to Makkah by Qays bin Mashar Saidawi, Abdur Rahman bin Abdullah bin Shaddad Arhabi and Ammāra bin Abdullah Saluli. Two more days passed and then the supporters of Kufa sent a letter to the Imam through Hāni bin Hāni Sabi'i and Sa'id bin Abdullah Hanafi. The following are the contents of the letter:
In the name of Allah, the Beneficent, the Merciful
This is a letter to Husayn bin Ali from his Muslim and faithful supporters. Be quick and hurry up, for the people are waiting for you, and they do not look towards anyone other than you. Hurry up. Hurry up. We repeat: Make haste. Make haste. And peace be upon you."
Thereafter Shabath bin Rabie, Hajjar bin Abjar, Yazid bin Harith bin Ruwaym, Urwah bin Qays, Amr bin Hajjāj Zubaydi and Muhammad bin Amr Taymi wrote the following letter:
"The gardens and the deserts are green and fresh and the fruits have ripened. Proceed whenever you like. Your Iraqi soldiers are ready to receive you. And peace by upon you".
Letters from the Kufians piled up with the Imam and the messengers from Iraq gathered in Makkah. The Imam sent the following reply to the Kufians:
"In the name of Allah, the Beneficent, the Merciful.
From Husayn bin Ali to the believers and Muslims of Iraq. Hani and Sa'id, your last messengers, have brought your letters. I have read what you have written and pondered over it. You have written that you do not have an Imam and asked me to come to you so that Allah may perhaps draw you together on truth and guidance through me. Now I am sending to you my cousin Muslim bin Aqil in whom I have full confidence. If he writes to me that your elders and wise men confirm what your messengers say and what you have written in your letters I may proceed towards you soon. I swear by my life that a true Imam and leader is only he who takes decisions according to the Qur'an, establishes justice, promotes the Divine religion and dedicates himself to the path of Allah. And peace be upon you"
The Imam sent this letter through Hani and Sa'id. Then he ordered Muslim bin Aqil to proceed to Iraq along with Qays bin Mash'ar Saidawi, Ammara bin Abdullah Saluli and Abdul Rahman bin Abdullah Araji. He instructed him to tread the path of piety, to keep his mission secret, to be lenient and moderate, and to inform the Imam immediately if he found that the people supported the rising and revolution.
source : http://www.al-islam.org