The Prophet (Allah bless him and give him peace) said:
من كان يؤمن بالله واليوم الآخر فليقل خيرا أو ليصمت
“Whoever believes in Allah and the Last Day let him speak good or remain silent.”
[Bukhari and Muslim]
The Sufiya suggest that this applies to all types of talking. That is, of the tongue (speech) and that of heart & mind (active thinking)!