The Islamic Personality by Sheikh Taqīuddīn An-Nabahānī

The silence of the Prophet (SAW), i.e. His approval is from the Sunnah, it is like His saying and His action equally the same, so if one did an action in the presence of the Prophet, or in His era and He knew about it, and was able to forbid it, then He kept silent about it and approved it for him without dispraising his action; it will be looked, if there is no previous forbiddance for that action from the Prophet (SAW), and no forbiddance is known for it, then His silence about its doer, and His approval to him on it denotes the permissibility of that action and the lifting up of the blame for doing it. That is because, if his action is not permissible; the Prophet would forbid him, since the Messenger does not keep silence if He knows about an evil (munkar) action, therefore His silence is a daleel for the permissibility. If the Prophet had previously forbade that action and its forbiddance became known; the silence of the Messenger can not be imagined, because it would be an approval to do what is forbidden. And this is impossible for the Prophet (SAW). As for His silence about the kuffaar under the Muslims' protection (ahl al-dthimmah), who used to frequently go to their churches, which is a kufr action; it does not denote His approval to do the kufr, but it denotes leaving ahl al-dthimmah to perform their own worship, and it is not a daleel for the permissibility to go to the church. So the conditions to consider the silence to be from the Sunnah (approval) are: that there is no previous forbiddance for the action, and the Messenger (SAW) knew about it, i.e. it was done in His presence, or in His era and He knew it, and that the Messenger was able to forbid (prevent) it, except that the silence is not considered to be from the Sunnah. The meaning of the forbiddance it is restraining its doer, and not that the Messenger does not trend to it, because the dab lizard got eaten in the presence of the Prophet and He did not eat from it, and when He was asked He said:

"It did not exist in the land of my people so I find myself detests it" compiled by Al-Bukhari. So being that the Messenger detests it is not of the forbiddance, and His none restraining those who ate it is considered to be silence about it, so it is a daleel for the permissibility. Also it was narrated from Naafi': That Ibn Omar heard the sound of a shepherd's pipe, then he put his finger tips in his ears, and deviated his riding camel from the way while he was saying: O Naafi' could you hear? I say: yes, then he keeps going away until I said no, then he put his hands down, and returned his riding camel to the way, and said: I saw the Messenger of Allah (SAW) when He heard the sound of a shepherd's pipe and He did like this" compiled by Ahmad. This is not considered to be forbiddance for the shepherd, but it is a silence about him, and it is a daleel for the permissibility of the pipe and the permissibility of hearing it, and as for blocking His hearing; it is possible that He avoided it as He used to avoid many permissible things, as He avoided to keep in His house dirham or dinaar overnight, and as He avoided eating the lizard. Accordingly, the silence of the Messenger means not restraining the one who does the action, even if He shows His self detest of the action.