Assalaamu `Alaykum,
This quote is from al-Tusi in his al-`Uddah. I will quote the full passage:
وأما ما ترويه الغلاة ، والمتهمون ، والمضعفون وغير هؤلاء ، فما يختص الغلاة بروايته ، فإن كانوا ممن عرف لهم حال استقامة وحال غلو ، عمل بما رووه في حال الاستقامة وترك ما رووه في حال خطاءهم ( 1 ) ، ولأجل ذلك عملت الطائفة بما رواه أبو الخطاب محمد بن أبي زينب [ 1 ] في حال استقامته وتركوا ما رواه في حال تخليطه ،
وكذلك القول في أحمد بن هلال العبرتائي [ 2 ] ، وابن أبي عذافر [ 3 ] وغير هؤلاء . فأما ما يرويه في حال تخليطهم فلا يجوز العمل به على كل حال .
وكذلك القول فيما ترويه المتهمون والمضعفون وإن كان هناك ما يعضد روايتهم ويدل على صحتها وجب العمل به . وإن لم يكن هناك ما يشهد لروايتهم بالصحّة وجب التوقف في أخبارهم ، ولأجل ( 2 ) ذلك توقف المشايخ عن أخبار كثيرة هذه صورتها ولم يرووها واستثنوها في فهارسهم من جملة ما يروونه من التصنيفات ( 3 ) .
As for what is narrated by the Ghulat, the Accused Ones, the Weak ones, and others of these, of what is pertaining to the ghulat and their narrations, and if they are known of their straight condition and their ghuluww condition, you act of what is narrated in the straight condition and abandon what is narrated in their erring condition. As a result, the Ta'ifah have acted of what is narrated by Abu al-Khattab Muhammad b. Abi Zaynab in his straight condition and they abandoned what he narrated in his erring condition.
And the belief is like that in (narrations of) Ahmad b. Hilal al-`Abarataa'i, Ibn Abi `Udhaafar and others of these. And as for what they narrate in their erring condition, it is not permissible to act by it under any circumstance.
And the belief is like that in what is narrated by the accused ones and the da`eef ones. And if there are support for their narrations and it shows upon its authenticity, it is wajib to act by it. And if there are no shawaahid for their narrations to be authentic, then it is necessary to do tawaqquf (desist) in their reports. As a result that tawaqquf of (our) Shaykhs from many of their reports these portray it, and their lack of narrated it, and including it in their categlogs from all together what is narrated from the compilations
Source:
al-Tusi, al-`Uddah Fee Usool al-Fiqh, pg. 151