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وعن أبي هريرة رضي الله عنه أن رسول الله صلى الله عليه وسلم قال‏:‏ ‏ “‏من دعا إلى هدى كان له من الأجر مثل أجور من تبعه لا ينقص ذلك من أجورهم شيئاً، ومن دعا إلى ضلالة كان عليه من الإثم مثل آثام من تبعه لا ينقص ذلك من آثامهم شيئاً‏” –‏ ‏‏‏(‏رواه مسلم‏‏‏)‏‏ - The Messenger of Allah (ﷺ) said, “If anyone calls others to follow right guidance, his reward will be equivalent to those who follow him (in righteousness) without their reward being diminished in any respect, and if anyone invites others to follow error, the sin, will be equivalent to that of the people who follow him (in sinfulness) without their sins being diminished in any respect”. (Sahih Muslim)

Both the good and the bad outcome in this hadith apply. Many focus only on the good, and over-zealously speak on the Deen hoping to gain reward when in fact the sin of speaking without knowledge is serious and overlooked. It is a worse sin than shirk (as ibn al-Qayyim mentioned, due to what it involves of Shirk, the beginning of Shirk being speaking about Allah and thinking about Allah without knowledge, and what it leads to and involves of many aspects of Shirk & Kufr), in his commentary on the ayah:

قل إنما حرم ربي الفواحش ما ظهر منها وما بطن والإثم والبغي بغير الحق وأن تشركوا بالله ما لم ينزل به سلطانا وأن تقولوا على الله ما لا تعلمون - Say, "My Lord has only forbidden immoralities - what is apparent of them and what is concealed - and sin, and oppression without right, and that you associate with Allah that for which He has not sent down authority, and that you say about Allah that which you do not know." (7:33)

For more on this: https://abdulmaliklive.wordpress.com/category/dawah-calling-to-truth/