"When a half of Shaban remains then do not fast."
[Tirmidhi 738]
Sheikh Ibn Baaz رحمه الله said,
"This is a sahih hadith as Shaykh Nasir ad Deen al Albani رحمه الله said. What is meant is that it is not allowed to start fasting after halfway through the month. But if a person fasts most or all of the month, then he is following the Sunnah."
[Majmoo' Fataawa 15/385]
Sheikh Ibn Uthaymeen رحمه الله said,
"Even if the hadith is sahih, the prohibition in it does not mean that this is haram, rather it is simply makruh, as some of the scholars have understood it to mean. But whoever has the habit of fasting regularly should fast, even if it is after halfway through Shaban."
[In his commentary on Riyad al Salihin 3/394]