The Messenger of Allah ﷺ as saying: Do not fast one day or two days just before Ramadan, except in the case of a man who has been in the habit of observing the particular fast, for he may fast on that day.  

عَنِ النَّبِيِّ ﷺ قَالَ لاَ تَقَدَّمُوا صَوْمَ رَمَضَانَ بِيَوْمٍ وَلاَ يَوْمَيْنِ إِلاَّ أَنْ يَكُونَ صَوْمًا يَصُومُهُ رَجُلٌ فَلْيَصُمْ ذَلِكَ الصَّوْمَ  

She never saw the Prophet ﷺ fasting the whole month except Sha'ban which he combined with Ramadan.  

عَنِ النَّبِيِّ ﷺ أَنَّهُ لَمْ يَكُنْ يَصُومُ مِنَ السَّنَةِ شَهْرًا تَامًّا إِلاَّ شَعْبَانَ يَصِلُهُ بِرَمَضَانَ